1. Field of the Invention
The invention relates to an efficient F.sub.2 -laser, and particularly to an F.sub.2 -laser that exhibits improved performance at 157 nm. This improvement is achieved by using neon as the primary buffer gas and by operating the gas at a higher temperature.
2. Discussion of the Related Art
A particular type of gas discharge laser is the F.sub.2 -laser having an advantageous emission spectrum including one or more lines around 157 nm. This short wavelength, or high energy, (157 nm=around 7.9 eV) photon emission is advantageous for photolithography applications because the critical dimension (CD), which represents the smallest resolvable feature size producible using photolithography, is proportional to the wavelength. This permits smaller and faster microprocessors and larger capacity DRAMs in a smaller package. The 7.9 eV photon is also readily absorbed in high band gap materials like quartz, synthetic quartz (SiO2), Teflon (PTFE), and silicone, among others, such that the F.sub.2 -laser has great potential in a wide variety of material processing applications. The construction and electrical excitation of the F.sub.2 -laser differs from that of another type of gas discharge laser known as the excimer laser. One difference is that the laser gas of an excimer laser includes a laser active constituent gas which has no bound ground state, or at most a weakly bound ground state. The laser active gas molecule of the excimer laser disassociates into its constituent atomic components upon optical transition from the upper to the lower state. In contrast, the laser active gas constituent molecule (F.sub.2) of the F.sub.2 -laser responsible for the emission around 157 nm is bound and stable in the ground state. In this case, the F.sub.2 molecule does not disassociate after making its optical transition from the upper to the lower state.
The F.sub.2 -laser has been known since around 1977 [see, e.g., Rice et al., VUV Emissions from Mixtures of F.sub.2 and the Noble Gases-A Molecular F.sub.2 laser at 1575 angstroms, Applied Physics Letters, Vol. 31, No. 1, Jul. 1, 1977]. However, previous F.sub.2 -lasers have been known to exhibit relatively low gains and short gas lifetimes. Other parameters such as the pulse-to-pulse stabilities and laser tube lifetimes have been unsatisfactory. In addition, oxygen and water exhibit high absorption cross sections around the desired 157 nm emission line of the F.sub.2 -laser, further reducing overall efficiency at the wafer when encountered by the laser beam anywhere along its path. To prevent this absorption, one can maintain a purged or evacuated beam path for the F.sub.2 -laser free or relatively free of oxygen and water, however costly and burdensome it may be for the operator. In short, despite the desirability of using short emission wavelengths for photolithography, F.sub.2 -lasers have seen very little practical industrial application to date. It is desired to have an F.sub.2 -laser with enhanced gain, longer pulse lengths and pulse-to-pulse stability, and increased lifetime.
The VUV laser radiation around 157 nm of the F.sub.2 -molecule has been observed as being accompanied by further laser radiation output in the red region of the visible spectrum. This visible light originates from the excited fluorine atom (atomic transition). It is desired to have an F.sub.2 -laser wherein the output in the visible region is minimized in order to maximize the energy in the VUV region.
Although the active constituent in the gas mixture of the F.sub.2 -laser is fluorine, the amount of pure fluorine amounts to no more than about 5 to 10 mbar of partial pressure within the gas mixture. A higher overall pressure is needed to sustain a uniform discharge. Consequently, a buffer gas is needed to raise the discharge vessel pressure, typically to well above atmospheric pressure (e.g., 3-10 bars or more), in order to achieve a clean discharge and realize an efficient output of the 157 nm radiation.
For this reason, F.sub.2 -lasers have gas mixtures including an inert buffer gas of typically helium. When helium is used, however, the output in the red visible region can range to about one to three percent of the VUV emission. In addition, the VUV pulse lengths tend to be relatively short. As noted above, it would be desirable to minimize the visible output as well as to increase the length of the VUV output pulses in order to improve both line selection and line narrowing capability.